1. Field of the Invention
This invention relates to methods and apparatus for drilling boreholes, and more particularly relates to methods and apparatus for drilling boreholes by means of a drill bit driven by a downhole motor.
2. Description of the Prior Art
The continuing search for oil and gas reservoirs has resulted in the drilling of boreholes to greater depths than had previously been conventional. At depths of 10,000 feet and below, the drilling capabilities of conventional rotary drilling rigs are often limited because the greater length of the tubular drill string required severely limits the rotational speeds at which the drill string and bit can be rotated without overstressing the drill string. The risk of breaking the drill string and the resulting down-time to recover the severed pipe have typically made the operators reluctant to operate at rotational speeds above about 50 revolutions per minute (r.p.m.).
However, at such low rotational speeds, relatively low formation penetration rates result with the use of diamond drill bits, which bits are ordinarily preferred in order to minimize the frequency of replacement. In fact, diamond drill bits generally achieve maximum economic penetration rates at speeds between about 400 and about 800 r.p.m. which speeds are not practical with conventional rotary rigs. This problem has been avoided to some extent by the use of a downhole motor to rotate a full-sized drill bit at high speeds.
Other problems encounted in the rotary drilling of boreholes with conventional apparatus and methods include the tendency of most full-sized drill bits to cut cores due to the relatively slow speed at which the centermost cutting elements of the drill bit move as compared to the relatively high speed of the outermost cutting elements. When the drill bit is rotated about a fixed axis relatively minor damage to the drill bit can result in the excessive wearing of a ring of cutting elements, thereby preventing further penetration and requiring replacement of the drill bit before full use can be made of all the cutting elements. Thus a need exists for an improved apparatus and method for drilling boreholes.
Accordingly, it is a primary object of this invention to provide an improved apparatus and method for drilling boreholes which overcome the aforementioned deficiencies of the conventional drilling apparatus and methods.
Another object of this invention is to provide an improved drilling apparatus and method in which the drill bit can be rotated at a higher speed than the drill string, thereby allowing the selection of a bit speed to maximize the formation penetration rate while simultaneously reducing the stress on the drill string.
Yet another object of this invention is to provide an improved apparatus and method in which substantially the entire face of the bottom of the borehole is subjected to cutting elements traveling at relatively high speeds.
Still another object of this invention is to provide an improved apparatus and method for drilling boreholes in which the tendency to cut a core is avoided.
A further object of this invention is to provide an improved drilling apparatus and method in which the cutting elements of the drill bit are subjected to a more uniform rate of wear so as to extend the useful life of the drill bit.
A still further object of the invention is to provide an improved drilling apparatus and method in which the rate of formation penetration is increased by the use of higher drill bit rotational speeds applied uniformly across the entire face of the bottom of the borehole.
Still other objects, advantages and features of the invention will become apparent to those skilled in the art from the following description when taken in conjunction with the accompanying drawings.